Biomedical Engineering Reference
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action chaotically mixes the solutes in each droplet. A liquid-liquid
droplet microluidic mixer has been used to rapidly mix luorophore
molecules with silver nanoparticles to enhance near-infrared
luorescence.
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This microluidic platform can ine tune the salt
concentration, colloid concentration, and mixing eficiency to avoid
nanoparticle aggregations.
7.5.3
Immobilizing a Catalytic DNA Molecular Beacon
on Au Nanoparticle to Detect Pb(II) Species
The strong absorption of thiol groups on gold surfaces makes it
easier to tag enzymes on gold nanoparticles when enzyme molecules
are modiied with a -SH group for immobilized enzymatic assays.
Sweedler and colleagues immobilized one thiol derivative of a
Pb(II)-speciic DNAzyme luorescent sensor on gold nanoparticles.
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In their method, a self-assembled monolayer of thiolated DNAzyme
irst forms when enzyme strands are strongly absorbed on gold
nanoparticles. This immobilized DNAzyme monolayer is then
hybridized with complementary luorophore-containing substrate
solutes. As soon as they react with Pb(II), the substrate strands are
cleaved to liberate a luorescent fragment that indicates the existence
of lead ions. The linear relationship between luorescence intensity
and Pb(II) concentration spans four decades (1 nM to 10 μM). The
detection limit of this immobilized assay is 1 nM, which is a full order
of magnitude lower than that of the previous solution-based method.
The immobilized DNAzyme is able to regenerate after cleavage to
allow multiple sensing cycles. This immobilized enzymatic assay
can be applied to a stand-alone portable device when the enzyme
immobilized nanoparticles are reserved in the nanopore cavities of
a microluidic-nanoluidic hybrid multilayer device.
7.6
Other Detection Techniques in Microfluidic
Devices Using Noble Metal Nanoparticles
7.6.1
Microfluidic Devices to Facilitate Surface-
Enhanced Raman Scattering Detections
When an incident light beam interacts with a polarizable molecule,
the induced molecular dipoles oscillate with the light frequency and
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